1. Field of the Invention
The present invention generally relates to electrical utility meters, and more particularly, to electrical utility meters suitable for use in polyphase power configurations to detect power quality disturbances.
2. Description of the Related Art
Most electrical utility service providers, or simply utilities, generate polyphase electrical power, and typically three phase power. Polyphase electrical power is alternating current (AC) electrical power that is supplied on a plurality of power supply lines wherein the voltage waveform on each of the power supply lines has a unique phase angle. Polyphase electrical power may be provided to customers in a plurality of wiring configurations. A wiring configuration is defined by the number of wires (e.g., three wire or four wire) and the wiring relationship between the phases (e.g., wye or delta). While only a single phase of the polyphase electrical power may typically be provided for single family residences, true polyphase electrical power is typically provided to larger facilities such as commercial and industrial facilities.
The alternating current (AC) electrical power delivered to residences, commercial facilities and industrial facilities ordinarily approximates a sine wave with respect to time, and typically flows through a residence or facility as an alternating current (AC) that also approximates a sine wave with respect to time. Utilities operate most efficiently and safely when both the voltage and current are sine waves. However, certain types of loads draw current in a non-sinusoidal manner, and if these loads are large relative to the utility""s source impedance, the voltage can become non-sinusoidal as well.
These non-sinusoidal voltage and current waveforms may be conveniently expressed as a Fourier series (i.e., a sum of sinusoidal waveforms of varying frequency, amplitude, and phase angle). Under most circumstances, the Fourier series for a utility includes a fundamental frequency, typically 50 or 60 Hertz, plus integer multiples of the fundamental frequency. These integer multiples of the fundamental frequency are referred to as xe2x80x9charmonics.xe2x80x9d
Instruments for measuring alternating current (AC) power system voltage and current harmonics are well known to those skilled in the art. With the growing popularity of non-linear electrical power loads such as adjustable speed drives, personal computers, arc furnaces and the like, it is often desirable to determine and record whether the level of voltage or current harmonics exceed some limit.
Commercially available instruments such as the Basic Measuring Instruments model 3030 or 3060 can be equipped with a feature that allows a user to program thresholds for voltage and/or current harmonics, and receive an alarm if these thresholds are exceeded. While this type of feature may have benefits in some instances, it fails to provide certain significant functions. For example, such conventional instruments do not provide any guide or pattern (i.e., template) to which the harmonic content of a measured signal may be compared to determine whether equipment at a particular site is susceptible to electrical damage or interference. This type of function is particularly important for sites having loads such as motors, rotating machines and communication equipment that are prone to damage or interference in the presence of specific harmonic conditions. In particular, the use of templates having defined harmonic thresholds enables a user to select a template for their specific type of concern, and if desired, adjust the thresholds to further customize the meter to a particular site. The present invention addresses this and other issues.
The present invention provides an electrical meter for monitoring electrical conditions in a polyphase electrical system. According to an embodiment, the meter generates measured harmonic data for a plurality of phases in the polyphase electrical system. A user may select at least one of a plurality of predetermined templates including one or more harmonic data thresholds. Each of the predetermined templates may correspond to a condition where a specific device is susceptible to electrical damage or interference. The measured harmonic data is compared with the one or more harmonic data thresholds included in the at least one selected predetermined template. An alarm is generated when the measured harmonic data exceeds the one or more harmonic data thresholds included in the at least one selected predetermined template. According to a preferred embodiment, the alarm is generated in dependence upon the specific template whose one or more harmonic data thresholds are exceeded.
According to another embodiment, the meter generates measured harmonic data for a plurality of phases in the polyphase electrical system. The measured harmonic data is compared with harmonic data thresholds included in a plurality of predetermined templates. Each of the predetermined templates may correspond to a condition where a specific device is susceptible to electrical damage or interference. An alarm is generated when the measured harmonic data exceeds at least one harmonic data threshold included in the plurality of predetermined templates. According to a preferred embodiment, the alarm is generated in dependence upon the specific template whose at least one harmonic data threshold is exceeded.